Replace Deref impl on RsaPrivateKey with AsRef

The `RsaPrivateKey` type previously had a `Deref` impl providing access
to the associated `RsaPublicKey`.

`Deref` is intended for "smart pointer types", i.e. container types
which manage a (typically generic) inner type in some way. This doesn't
seem like one of those cases.

`AsRef`, on the other hand, is for cheap reference conversions, which is
exactly what's happening here, so it's a better fit and provides the
same functionality (albeit explicitly rather than via deref coercion).

src/key.rs

@@ -57,6 +57,12 @@ impl PartialEq for RsaPrivateKey {
     }
 }
 
+impl AsRef<RsaPublicKey> for RsaPrivateKey {
+    fn as_ref(&self) -> &RsaPublicKey {
+        &self.pubkey_components
+    }
+}
+
 impl Hash for RsaPrivateKey {
     fn hash<H: Hasher>(&self, state: &mut H) {
         // Domain separator for RSA private keys
@@ -73,13 +79,6 @@ impl Drop for RsaPrivateKey {
     }
 }
 
-impl Deref for RsaPrivateKey {
-    type Target = RsaPublicKey;
-    fn deref(&self) -> &RsaPublicKey {
-        &self.pubkey_components
-    }
-}
-
 impl ZeroizeOnDrop for RsaPrivateKey {}
 
 #[derive(Debug, Clone)]